Well, it almost certainly evolved to make the moth mimic a spider, presumably a predator. As the website says, somewhat breathlessly:

Now this is an example of mimicry at its finest! This newly discovered species (2005) of moth dubbed the Lygodium Spider Moth (Siamusotima aranea) is so named for its preference of feeding on Lygodium species, an invasive Old World climbing fern, and has markings on its wings that make it look just like a spider with orange, spindly legs! This moth mimics a spider so well that I couldn’t even tell what it was at first when I saw the picture from far away!

Why would a moth evolve to mimic a spider? Earlier research on insects (see below) suggests that when prey like this sense an approaching spider, they display their wings (as in the photo below), and that deludes the spider into thinking it’s encountered another spider of its own species. That would be an aggressive conspecific encounter—one from which spiders often flee. A moth with this pattern, then, might escape predation by actually driving away the predator, or at least distracting it for long enough to allow the moth to fly away unharmed.

This evolutionary scenario is speculative, for as far as I know it hasn’t been tested (or even observed) in the moth shown below, but I don’t see any other explanation for the wing pattern.

Note that the patterns on the moth give it eight legs, just like a spider!

A paper in Science in 1987 by Mather and Roitberg (no free link, but reference below) shows something similar: patterns on a “true fruit fly” (the tephritid snowberry fly, Rhagoletis zephyria). For a long time these wing patterns were thought to be simply species-recognition marks, until clever zoologists realized that they looked like something else.

This is another rare case of a prey actually mimicking its predator—in this case a jumping spider (the authors used the zebra spider Salticus scenicus for their tests). When the fly senses something approaching, it spreads its wings a bit and wobbles from side to side, much like the gait of a jumping spider. This makes an approaching jumping spider think that the fly is actually a conspecific spider. And the spider, sensing an aggressive encounter on tap, usually flees. The displaying fly is saved.

Experiments show that spider fled from displaying flies at rates similar to which they fled from other spiders. Further experiments that obliterated the flies’ wing patterns with ink showed that this made them more susceptible to predation (predators didn’t flee as often, but pounced), although there were no controls for the effect of ink-painting on the flies’ well being and vigor.

Here’s what the predator and the tephritid fly look like:

Finally, a new paper in Proc. Nat. Acad. Sci. USA (free download; reference below) by Sonja Wedmann et al. describes the oldest fossil leaf-mimikcing insect known, a phasmid found in 47-million-year-old deposits in Germany. The fossil is a male, remarkably preserved, and has been given the name Eophyllium messelensis. (Note: if you can’t see a fossil picture directly below, click on the icon.)

The fossil strongly resembles some modern leaf phasmids, like the modern species Phylllium celebicum (below). Note that the female, on the left, is a much stronger leaf mimic than the male (right). According to the authors, this sexual dimorphism is quite common in phasmids.

Why this ubiquitous dimorphism? It resembles some types of Batesian mimicry in butterflies, in which edible females evolve to resemble both males and females (who look alike) of a toxic or distasteful model species, but the edible males don’t show mimicry, resembling the ancestor. In both cases, I suspect, the explanation is the same: there would be an advantage to the males evolving mimicry, too, but an even larger disadvantage to changing their pattern if females have a fixed genetic preference for the ancestral type of male.

I’m not sure whether this explanation is correct, and don’t even know if it’s been tested, but it is at least conceptually testable and makes some sense in light of what we know about sexual dimorphism (females have strong preferences for certain types of males).

The mimicry does show, though, that there were visually-hunting predators around then, for why else would these phasmids evolve mimicry? Wedmann et al. posit that the predators may have included birds, primates, and bats, all of them known from the same fossil deposits.

13 Comments

If it has the right number of legs then surely means there was an evolutionary pressure to have the correct number of legs. Which would suggest an incorrect number of legs wasn’t a convincing enough mimic. So, by extension, does that suggest spiders have, to some degree, an ability to count? (At least up to 8?)

Beautiful moth picture ladyatheist! I don’t think I have ever noticed before (or seen) that the center of the eye spots are translucent. So as the moth wing moves the image/colors seen through the eye spot would change? It would be interesting to understand how the predators see that.

Thank you for this! The Spider moth is amazing.
I have been collecting bookmarks on moths and butterflies that mimic a predator. In some cases, I wonder if the mimicked animal is actual the predator of what the moth is actually trying to scare away. In other cases I wonder if the animal that is being mimicked has long since been extinct – especially in cases where the it may be mimicking a lizard.
In one case an owl (with wings spread out) may be mimicking the face of a rather large raccoon type face. I stepped across the room and there it was, a raccoon like face. So go figure! And regarding evolution…in these mimicry case, I think they are too specific to be by natural selection, but I also believe we can scientifically figure this out – but what is making these specifically beautiful mimicked illusions – are bugs and bats just a lot smarter than we categorize them? C

“Earlier research on insects (see below) suggests that when prey like this sense an approaching spider, they display their wings (as in the photo below), and that deludes the spider into thinking it’s encountered another spider of its own species.”

I’m not so sure that this idea has any merit, as spiders are cannibalistic.

It does hold some merit, spiders are cannibalistic but more often than not they will flee from another spider given the chance. I;d say for every one act of spider cannibalism I’ve seen, over ten others have done a runner!

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[…] way as to give them the appearance of an 8-legged threat. But these flies put on a grand charade, using their entire bodies to fool predators into backing down, not just the tips of their wings. Additionally, the spider mimicking flies are in a totally […]